This invention relates to ignition systems for the firing of electrically actuable igniters of the type used as electric fuseheads in blasting detonators and for the igniting of incendiary charges in pyrotechnic devices. Specifically, this invention relates to an ignition system for the firing of such electrically actuable igniters and to methods for assembling such systems and to methods for firing electrically actuable igniters.
Electrically actuable igniters generally include a resistive electric ignition element having two electrical connection terminals. A two conductor igniter leading wire is usually connected to these two electrical connection terminals so that just prior to use, the two conductor igniter leading wire can be connected to a source of firing energy. The resistive electric ignition element is a conducting composition that is electrically heated to an ignition point or includes a bridge wire element having a predetermined resistance for generating heat in thermal contact with an incendiary composition. The construction and use of electrically actuable igniters commonly used as the fuseheads of blasting detonators are described and illustrated in Blasting Practice published by ICI-Nobels Explosive Company Limited (Kynoch 1972), Chapters 2 and 3.
In the igniter discussed, the resistive electric ignition element includes a bridge wire that is metallurgically bonded across the pair of metal electrodes, to which ignition leading wires are also bonded. The electrodes are embedded in an incendiary composition. The igniter leading wires are 0.51-1.22 mm. in diameter copper or iron wires insulated with a synthetic plastic material such as polyvinyl chloride.
Specifically, three groups of resistive electric ignition elements having different sensitivities have been developed for electric detonators:
Group 1 ignition elements have a characteristic resistance of 0.9-1.6 ohms and a firing sensitivity in the range of 3-5 millijoules/ohms;
Group 2 ignition elements have a characteristic resistance of 0.15-0.18 ohms and a firing sensitivity in the range of 80-140 millijoules/ohms; and
Group 3 ignition elements have a characteristic resistance of 0.02-0.04 ohm and a firing sensitivity in the range of 1,000-2,500 millijoules/ohms.
Commercial electric detonators are generally supplied with two separate leading wires which, to facilitate later connection to a source of firing energy are barred to a length of about 1 cm. at their unconnected ends. In use, it is understood that the barred ends of the leading wires are connected by twisting a barred end of a leading wire from one ignition element with a barred wire end form another igniter to connect the igniters in series, parallel or series-parallel arrangement to the source of electrical firing energy. In many cases igniters arrive from a manufacturer with the two barred wire ends from each igniter twisted together and sheathed by the manufacturer as a safeguard against extraneous electrical sources. In these cases, the barred ends have to be separated by the user.
When the wires are unsheathed the detonator is subject to the risk of accidental ignition by extraneous electrical energy sources such as an electrostatic discharge from a person or from a substance in close proximity to the igniter such as pneumatically loaded ammonium nitrate fuel oil explosive (ANFO), a stray current from a battery or electric line, or a stray galvanic current. In addition to this serious safety problem, the use of these commercially available igniters is highly inconvenient. The actual connection of the leading wires is tedious, difficult and time consumming, especially for parallel or series-parallel circuits. Connections must often be made in the poor light and confined space of an underground mine or tunnel. In such an environment, there is always the possibility of a faulty connection or of bare wire connections coming into contact with water or other good earth leakage contact prior to the intended firing of the igniters.
Various arrangements have been attempted to protect igniters from stray current and reduce the safety hazard. Igniters have been coupled to a firing circuit through a transformer core and, in some igniters assemblies designed for military use, the cores of the transformers have been separable into two parts to allow the primary and secondary circuits to be kept separate until assembly was required for firing. Then, the assembly was armed for use by coupling the transformer core portions to one another. In these military igniter assemblies the transformer windings were separate from the leading wires of the igniter and also separate from the firing cable coupled to the power supply. If used with a commercial igniter the transformer windings would have to be connected to the firing cable and to the igniter leading wires manually. Such a situation remains unsatisfactory.